Residential doors are typically constructed either of wood or steel which encases wood laminate. The doors are either pre-hung or installed on site on wood door jambs. The door jambs are normally about 45/8" wide and about 3/4" thick. Pre-hung doors are installed in the rough door openings and are shimmed to a level and plumb condition by installing shims between the door jamb and the wall framing which surrounds the door opening. The gaps that receive the shims are typically about 1/8" to 3/16" wide, so the jamb is essentially suspended in the rough door opening with a gap around it containing the shims. The gaps are eventually covered up by trim such as door casing or moldings.
Many efforts have been made to enhance the security of entry doors in order to reduce forced entries. Steel laminate doors can be used to replace wood doors in order to increase the strength of the door itself. The bolt faces and sides can be equipped with wrap around metal covers in order to enhance their strength. The lock sets normally have sufficient structural integrity that they seldom fail during attempted break ins. Most often, successful forced entries result from the destruction of the door material surrounding the lock set and/or destruction of the jamb in the area which receives a dead bolt or other locking bolt. The vulnerable area of the door around the lock set can be adequately strengthened in a number of ways. However, due to space limitations and the lack of structural support resulting from the presence of a gap around the door jamb, attempts to significantly strengthen the jamb have not been successful.
The entry door is kicked inwardly during forced entry, and the force that is applied to the door is transferred to the dead bolt. The latch bolt is not really intended as a security device and projects only about 1/2" into the jamb, so it can be easily dislodged. The dead bolt is intended to provide security and usually projects about 1" so that it extends completely through the 3/4 thick jamb. However, the dead bolt passage in the jamb is set back from the inside edge of the jamb by only about 1/2". Although a strike plate is normally installed on the jamb around the bolt passage, the strike plate is only 1/16" thick and is mounted with small screws about 3/4" long.
When the door is forcefully kicked inwardly, the dead bolt acts as a lever against the jamb in the small area (3/4" thick and about 1/2" wide) between the bolt passage and the inside edge of the door jamb. The force the bolt applies can easily destroy this part of the jamb and tear the strike plate away. The vulnerable area of the door jamb is made even weaker by the gap between the jamb and the solid wall framing, and also by the practice of locating the edge of the jamb flush with the finished wall. This practice results in the vulnerable area of the jamb being backed only by relatively weak wallboard rather than stronger framing.
Attempts that have been made to enhance the strength of the jamb have been unsuccessful. Providing long screws for the strike plates and/or strengthening the strike plates does not result in significant increased support for the vulnerable part of the jamb. The bolt still acts as a lever which can force the strike plate against the jamb and destroy the 1/2" of the jamb between the bolt and the jamb edge. Interior devices such as sliding bolts, hasps and security chains are often used, but these types of devices can be torn loose either from the door or from the surface around the door on which they are mounted.